The invention relates to a method of slag foaming a molten slag which in the production of non-stainless steel in an electric arc furnace is present on the steel melt, wherein shaped pieces, such as pellets or briquettes are added to the slag, and wherein the contents of the shaped pieces react with the Fe-oxides in a reducing manner by splitting off gas, and wherein the reaction gases that are produced lead to slag foaming.
When operating electric arc furnaces, solid materials which have been filled in, primarily scrap and alloys, are melted using the light arcs of the electrodes which project from above into the furnace vessel. In this case, in addition to its primary function which is to remove undesired components from the melt, the slag has a protective function because it partially fills out the space between the electrode ends and the metal surface and the refractory lining of the furnace protects against the radiation energy of the electric arc. This protective function of the slag can be improved by causing foaming of the slag by suitable methods.
During melting of the solid material in an electric arc furnace for manufacturing non-stainless steel, a slag is formed which contains a high proportion of metal oxides, primarily of Fe-oxide. The concentration of the iron oxide frequently reaches values greater than 20%.
The metallurgy of the process of such slags produces the following partial reactions which take place successively:{O2}=2{O}Thermal dissociation of the oxygen[Fe]+[O]═(FeO)Iron oxidation in the melt[C]+(FeO)═[Fe]+{CO}Iron oxide reduction at the phase border slag/metal
The last reaction is of fundamental importance for the manufacture of carbon steel because iron oxide is the most important component in the foamed slag formation. If the slag viscosity is suitable for maintaining the foam, the simple blowing in of carbon and oxygen into the slag causes foaming, wherein the gaseous CO with its bubbles additionally intensifies the slag foaming, the gaseous CO being formed by the reduction process of the metal oxide with the carbon.
Decisive for the foam slag formation are the components of the added foam material as well as the slag viscosity which, in turn, depends on the composition and the temperature of the molten slag. Primarily, the viscosity defines a range of the state of the molten slag in which a foam formation is possible. Therefore, the control of the slag basicity responsible for the viscosity is important, whereby the produced gas bubbles are forced to temporarily remain in the slag layer. The limestone added in the foaming material constitutes another gas source because the thermal dissociation releases this material in accordance with the following equation CO:(CaCO3)═(CaO)+{CO}
The phenomenon of the bubble formation constitutes a process which utilizes the mechanical force of the reacting gas bubbles for producing a new surface area in the slag. The buoyancy forces acting on the gas bubbles split the slag surface temporarily and saturate the complete slag layer for producing the foam. In the case of a protracted gas flow from the reacting materials, the number of the accumulating bubbles increases with the increasing foam. As a consequence, the height of the foam layer increases with a growing quantity of gas; the quantity is directly proportional to the quantity of the foam material.
Important in such a mechanism is the optimum placement of the reaction substances in order to obtain a maximum foaming effect in this manner. The optimum placement takes place in the border area between the slag layer and the liquid metal.
In the manufacture of non-stainless steel, the slag foaming has in the past conventionally been started by means of blown-in carbon (coke, graphite, coal) and oxygen. This technology requires a complicated maintenance of blow-in systems and the use of oxygen and carbon carriers. This technology is not only complicated, but is also not very efficient in relation to the introduced components, because dusts which have been blown in (including Fe2O3) are in the case of an incorrect blow-in angle for the most part removed from the reduction process by the furnace suctioning system.
EP 0829 545 B1 describes a method for manufacturing a foam slag on molten stainless steel in an electric furnace, wherein into the slag is introduced a powder by an injection medium, for example, nitrogen, which is composed of a metal oxide, either zinc oxide or lead oxide, and carbon. The oxide contained in the powder is reduced by reacting with the carbon. This causes the formation of bubbles in the slag which are composed essentially of carbon monoxide and cause the slag to foam. Because the added powder has a relatively large surface area, a short vigorous reaction with the slag occurs which additionally takes place locally limited in the vicinity of the injection or blow-in device in the melt bath.
In order to avoid the disadvantages of introducing pulverous substances, it is proposed, in WO 2004/104232, for manufacturing a foamed slag on a high chromium containing steel melt, to add the materials used for foaming the slag, as a mixture of metal oxide and carbon, as shaped pieces which are compressed and/or provided with a binding agent into the electric furnace. The density of these shaped pieces is adjusted in such a way that they float and react in the metal and near the phase border melt/slag.
In DE 10 2007 006 529 A1, in manufacturing a foamed slag on a high chromium-steel melt additionally the metal oxides present in the slag, primarily chromium oxide, are reduced by the briquettes or pellets floating near the phase border melt/slag, wherein the produced reaction gases intensify the slag foaming. For this purpose, the briquettes or pellets added to the electric arc furnace are composed of a defined mixture of an iron carrier as a ballast material of carbon or carbon and silicone as a reduction agent as well as a binder.